The KATRIN experiment aims at the direct measurement of the electron neutrino mass with $\,0.2\,$eV/c² sensitivity. The high luminosity windowless gaseous molecular tritium source together with the magnetic adiabatic collimation of the electrostatic (MAC-E) filter technique allows for precision endpoint spectroscopy of the tritium beta decay. The analyses of the first and second tritium campaign yield an upper limit of $m_\nu < 0.8\,$eV (90% C.L.) (Nature Physics 18 (2022) 160).
Despite the advances in lowering the background rate, e.g. by implementation of the shifted-analysing-plane mode (arXiv:2201.11743), further background reduction measures are required.
The background is assumed to mainly consist of electrons from the de-excitation of highly-excited Rydberg atoms within the main spectrometer, which originate from alpha-decays in the spectrometer walls. Their kinetic energy at the detector is indistinguishable from tritium beta decay electrons, but that is not the case for their pitch angle distribution.
We introduce research and development of the active transverse energy filter (aTEF, arXiv:2203.06085) as a concept that allows to discriminate electrons at the detector based on their pitch angle and can differentiate between signal and background electrons in KATRIN.
We will discuss our investigations of first prototypes, as well as the development steps necessary to construct a reliable test setup using microstructured silicon-based aTEF detectors.
We acknowledge the support of Helmholtz Association (HGF); Ministry for Education and Research BMBF (05A17PM3, 05A17PX3, 05A17VK2, 05A17PDA, 05A17WO3, 05A20VK3, 05A20PMA and 05A20PX3); Helmholtz Alliance for Astroparticle Physics (HAP); the doctoral school KSETA at KIT; Helmholtz Young Investigator Group (VH-NG-1055); Max Planck Research Group (MaxPlanck@TUM); Deutsche Forschungsgemeinschaft DFG (Research Training Group grant nos. GRK 1694 and GRK 2149); Graduate School grant no. GSC 1085-KSETA and SFB-1258 in Germany; Ministry of Education, Youth and Sport (CANAM-LM2015056, LTT19005) in the Czech Republic; the Department of Energy through grants DE-FG02-97ER41020, DE-FG02-94ER40818, DE-SC0004036, DE-FG02-97ER41033, DE-FG02-97ER41041, DE-SC0011091 and DE-SC0019304; and the Federal Prime Agreement DE-AC02-05CH11231 in the USA. This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme (grant agreement no. 852845). We thank the computing cluster support at the Institute for Astroparticle Physics at Karlsruhe Institute of Technology, Max Planck Computing and Data Facility (MPCDF), and National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory.
We further acknowledge the Münster Nanofabrication Facility (MNF) and especially Banafsheh Abasahl and Alexander Eich for their support during the fabrication of silicon microstructure samples. We kindly thank Norbert Wermes (University of Bonn) and Peter Lechner (Max Planck Semiconductor Laboratory, Munich) for their valuable inputs.